The goal of a sustainable supply of clean and economical energy has stimulated intensive research in fuel cell technologies, which could provide electricity with high conversion efficiency and low environmental impact. Among all types of fuel cells, solid oxide fuel cells (SOFCs) are exceptional because they are less dependent on precious metal catalysts and are capable of using hydrocarbon fuels. One challenge for the commercial application of SOFCs is the high operating temperature which currently mandates the use of expensive packaging and interconnect materials. The enhancement of cathode activity is the bottleneck to reduce the operation temperature, because the oxygen reduction reaction (ORR) kinetics slow down exponentially as the temperature is decreased.
A good SOFC cathode needs both high oxygen exchange kinetics and high conductivity of both ions and electrons. To date, various cathode materials, including conventional La1−xSrxMnO3−δ, (LSM) and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), and newly developed Sm0.5Sr0.5CoO3−δ, (SSC), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), PrBaCo2O5+δ (PBC) as well as their derivatives, have been developed for SOFCs. Though the newly developed cathode materials show higher ORR activity, especially at intermediate temperature range, their unproven long-term stability and inadequate compatibility with electrolyte and other cell components, especially at the high temperatures required for fabrication, limit further application in SOFCs. The state-of-the-art cathode materials are still LSM (>800° C.) and LSCF (<750° C.). The main issue of LSM cathode is the extremely low oxygen ion conductivity in intermediate temperatures range (600-750° C.) which would limit ORR. For LSCF, the catalytic activity is likely limited by the surface catalytic properties. There is an additional concern about long-term stability of LSCF, due possibly to the change of surface state in LSCF. To counter these issues and develop intermediate temperature SOFCs, a surface modification technique has been developed to apply novel cathode catalytic materials to the well-established LSCF cathode material. The porous LSCF backbone serves as a “highway” for the transport of both oxygen ions and electrons because of its excellent ambipolar conductivity, while the surface functional layer could modify the surface properties of LSCF to offer higher electro-catalytic activity and greater stability. In previous studies, discontinuous coatings of SSC, La0.4875Ca0.0125Ce0.5O2−δ (LCC) and Ce0.8Sm0.2O1.9 (SDC) were all found to expedite the oxygen reduction on the surface due to their own high catalytic activity. Meanwhile, a conformal thin film coating of LSM enhanced the LSCF cathode stability through inhibition of SrO segregation.